The present invention relates to ion bombardment or implantation apparatus and more particularly to such apparatus having expedients which permit the control of surface potential of the target, particularly a target having a bombarded surface which is formed in whole or in part of an electrically insulative material.
Ion implantation is a technology of rapidly increasing importance in the fabrication of integrated circuits, particularly bipolar integrated circuits. In this bipolar technology, there is an increasing demand for (1) high dosage but relatively low time cycle implantation operations, and (2) ion implantation technology useful for introducing impurities through openings having at least one lateral dimension no greater than one mil. Since the implantation dosage is dependent on the combination of current and time, it follows that in order to achieve high dosage in relatively short time, the technology must move in the direction of high current ion implantation beams having currents greater than 0.5 ma. It has been found that when making such high current ion implantations of conductivity-determining impurities through electrically insulative layer openings having dimensions in the order of 0.1 to 1 mil. as is required in the high density, large scale integrated circuits, there is a substantial tendency toward the impairment or destruction of portions of this electrically insulative layer as well as exposed semiconductor areas resulting in potential short circuits which render the integrated circuit inoperative.
We believe that such impairment or destruction occurs due to an electrical breakdown of a potential built up on the insulating layer arising from the charge deposited by the positive ions which make up the primary ion beam. This potential buildup is particularly pronounced in high current beams which have a high density of positive ions. Without being bound on the theory involved, we believe that in such high current beams, positive ions have such a high density that the floating cloud of electrons, which is inherently produced through the operation of the ion bombardment apparatus such as through secondary electron emission from material struck by the ion beam and neutral background gas ionization by the ion beam, is insufficient in quantity to fully neutralize the charge created by the positive ions on the target.
The concept of the positive ion beam and the effect of the secondary electron cloud is discussed in some detail in U.S. Pat. Nos. 3,997,846, 4,011,449 and 4,013,891, and in the article, "High Current Electron Scanning Method for Ion Beam Writing", W. C. Ko, at pp. 1832 - 1835, IBM Technical Disclosure Bulletin, Vol. 18, No. 6, November 1975, as well as in the text, "Ion Beams With Application to Ion Implantation", R. G. Wilson and G. R. Brewer, (John Wiley & Sons, New York, 1973) at pp. 132 - 143.
In addition, it appears that when the openings through which the ions are to be implanted have small lateral dimensions in the order of 1 mil, or less, secondary electrons which are normally produced by positive ions striking semiconductor substrate are minimized; this further contributes to the deficiency of available secondary electrons at the surface to neutralize the positive ion accumulation to prevent charge buildup. This effect is discussed in detail in copending application, Ser. No. 763,789, H. S. Rupprecht et al, filed Jan. 31, 1977.
While the problem described will arise in connection with such ion implantation through minute openings, similar problems may be expected to arise when the ion implantation is conducted with high current beams through thin regions in an electrically insulative layer over a semiconductor substrate rather than through openings in such an insulative layer.
The prior art has a suggested solution to this problem of charge buildup which involves directly irradiating the surface of the electrically insulative material with electrons in sufficient quantity to produce a negative potential on the surface of the insulative material sufficient to offset any positive charge produced by the ions in the beam. We have found that when such a direct impingement method is utilized, less than desirable effects are produced. First, the electron source is usually a heated filament, metallic member of some sort, or plasma. Such electron source can be adversely affected by material emitted from the target during ion bombardment and, in addition, these sources can give off material which can contaminate the target. In addition, since the electron source is conventionally a heated member such as a heated filament, the heat from the source will produce an undesirable heating effect at the target. Thus, if the target is covered with an electrically insulative material such as photoresist which is affected by heat, then the heated filament may damage the target.
In addition, since ion beam dosimetry, i.e., the measurement and control of ion beam current, is considered to be significant in ion implantation apparatus, there is the need in the art, particularly with high current beams, for a method and apparatus for controlling and minimizing the positive surface potential of the target which is compatible with dosimetry apparatus for measuring the beam current.